1. Field of the Invention
The present invention relates to a scanning line driving circuit used in, for example, an image display device or an electro-optical device such as an image sensor, and particularly, to a scanning line driving circuit that includes a shift register having a structure in which only electric field transistors of the same conductivity type are used and is capable of bi-directional scanning.
2. Description of the Background Art
An electro-optical device including a scanning line driving circuit that scans pixels connected to scanning lines is widely known. For example, in an image display device (hereinafter, referred to as a “display device”) such as a liquid crystal display device, a gate line (scanning line) is provided for each pixel row (pixel line) of a display element (display panel) in which a plurality of pixels are arranged in matrix, where display images are updated by sequentially selecting and driving the gate lines in a cycle of one horizontal period of a display signal. As a scanning line driving circuit (gate line driving circuit) for sequentially selecting and driving the pixel lines, that is, gate lines, there may be used a shift register that performs a round of shift operation in one frame period of a display signal.
Further, pixels of an imaging element used in an imaging device are disposed in matrix, and the data of the taken image is extracted by scanning those pixels by the gate line driving circuit. A shift register can be also used in the gate line driving circuit of the imaging device.
The gate line driving circuit using shift registers includes a plurality of cascade-connected shift register circuits provided, each of which is provided for each scanning line (gate line). Herein, each of a plurality of shift register circuits forming the gate line driving circuit is referred to as a “unit shift register”.
The shift registers used in the gate line driving circuit desirably consist of field effect transistors of the same conductivity type for reducing the number of steps in a manufacturing process for a display device. Therefore, for example, Japanese Patent Application Laid-Open No. 2004-157508 proposes a shift register consisting of n-type or p-type field effect transistors and a display device mounting the same.
FIG. 16 of Japanese Patent Application Laid-Open No. 2004-157508 shows an example of a gate line driving circuit that is driven by clock signals of two phases and is capable of bi-directional scanning. In the gate line driving circuit of FIG. 16, two dummy unit shift registers (dummy stage 0 and dummy stage 1) are used in addition to unit shift registers (SRC1 to SRC4) that actually drive gate lines. The dummy unit shift register (dummy stage 1) provided further downstream of the last stage (SRC4) is provided for resetting the last stage (SRC4) during scanning in a forward direction (from SRC1 to SRC4). The dummy unit shift register (dummy stage 0) provided further upstream of the first stage (SRC1) is provided for resetting the first stage (SRC1) during scanning in a backward direction (from SRC4 to SRC1).
As described above, in Japanese Patent Application Laid-Open No. 2004-157508, two dummy unit shift registers are provided in addition to the circuits that actually drive the gate lines, and accordingly the area required for forming the gate line driving circuit increases by an amount of the increase. An increase in area for forming the circuit is not preferred because it leads to an increase in cost of a display device.